The cells of higher eukaryotes acquire iron via the serum protein transferrin (Tf) and a high-affinity cell surface transferrin receptor (TfR). The expression of the TfR is highly regulated (greater than 20-fold) by iron availability with higher levels of expression occurring when iron is scarce. The regulation of the expression of the TfR is achieved by modulation of the level of the mRNA encoding the receptor. At least two genetic loci were found to participate in this regulation of TfR mRNA levels. A modest (2- to 3-fold) transcriptional regulation was found to be mediated by human genomic DNA 5' of the transcription start site. We have molecularly cloned and partially characterized the promoter of the human TfR gene. However, the TfR cDNA even when driven by a heterologous promoter remains highly regulated by iron. The TfR mRNA is 5 kb in length of which.approximately half is the 3' untranslated region (UTR). We have found the 3' UTR to be both necessary and sufficient as the major locus of iron responsiveness. We are attempting to understand the mechanism of regulation of TfR expression and to identify and characterize the elements of the 3' UTR that are responsible for this regulation. Among the sequence elements implicated to date are RNA stem-loop structures that resemble the iron-responsive element found in the mRNA of ferritin, another iron-regulated protein of cellular iron metabolism.